1. Field of the invention
This invention relates to heart pacing and more particularly to transvenous heart pacing leads.
For certain patients, reasonable success can be achieved with a heart pacing system which paces the ventricle at an externally imposed rate. In some patients, where the atrium is functioning adequately but is not capable of triggering the ventricle, an improvement in heart performance could be achieved if electrical activity in the atrium were sensed and the ventricle paced in synchronism with natural movement of the atrium. Pacing would then be provided at the correct physiological rate rather than at an externally imposed rate. In other patients, where the atrium is not functioning adequately, an improvement over straightforward ventricular pacing could still be achieved if the atrium and ventricle were paced sequentially so that advantage were taken of the pumping capability of the atrium.
The major practical disadvantage of atrio-synchronous pacing of the ventricle and atrioventricular-sequential pacing has been the requirement for separate atrio and ventricular leads. For this reason the techniques have not been widely employed.
2. Description of the prior art
It has been suggested, (The Lancet-Oct. 7th, 1978 pages 757 to 759) that an atrial electrode could be mounted on an otherwise conventional lead in the form of a small sleeve positioned approximately 13 to 18 cm from the ventricular electrode. With this arrangement, the sleeve electrode lies within the atrium when the ventricular electrode is in position and can be used to sense electrical activity in the atrium. There is--at best--intermittent contact with the atrial wall and although this arrangement is reasonably satisfactory for atrial sensing, the electrical contact with the atrium is insufficient to permit atrioventricular-sequential pacing.
It has further been suggested, (The Journal of Thoracic and Cardiovascular Surgery Volume 69 Number 4 April 1965 pages 575 to 578) that the atrial electrode could be mounted at the end of an atrial lead portion which branches from the main lead body. The atrial electrode is secured to the lead body during insertion of the lead and is released once the lead is in place by withdrawal of a guide wire to allow the atrial lead portion to take up its natural J-shape. This J-shape is intended to bring the atrial electrode into contact with the atrial wall and also serves to hinder retraction of the lead.
This lead has not become popular, probably because of difficulties in insertion and positioning. Even if the lead were correctly positioned with the atrial electrode in contact with the atrial wall, there is a risk of the electrical contact being impaired through subsequent movement of the atrial electrode. It will be appreciated that there is no natural "site" for location of the atrial electrode as there is with a ventricular electrode.